The invention relates to ion implantation and particularly to methods and applications for determining the alignment of the ion beam in the process chamber of an ion implanter.
Techniques of ion implantation are well known, particularly when used for implanting impurities etc. in semiconductor wafers, e.g. for obtaining a desired dopant concentration in regions of the wafer.
There is an increasing requirement for extreme accuracy between the direction of the beam of ions to be implanted in the wafer and the plane of the wafer itself, and more particularly the principle axis of the crystalline structure within the silicon wafer which is nominally normal to the wafer surface. An accurate knowledge and control of the angle of the ion beam to the surface of the wafer is necessary for obtaining precise implant angles relative to the wafer surface. This is important for so called shadow implants, that is implants into deep trenches on the wafer surface, and also for very shallow implants through narrow openings in a photo resist mask on the surface of the wafer. To achieve very high accuracy, it is necessary to have knowledge of the angle between the wafer and the ion beam to within fractions of a degree. Whilst mechanical calibration of the wafer holder of the ion implanter can permit control of the holder relative to the implantation process chamber to high accuracy, the direction of the ion beam within the process chamber may not be predictable to the same accuracy and may indeed be different for different process beams.
Rutherford Back Scattering is a well known technique for studying the surface composition and depth profiles of crystalline solids. The technique has also been considered for the measurement of dose accumulation in a semiconductor substrate during an implant. Prior art includes the book xe2x80x9cBack Scattering Spectrometryxe2x80x9d by Chu et al, published by Academic Press 1978. Reference may also be made to published Japanese Patent Specification 60-124343, published 3rd July 1985 (Hitachi).
A technique for checking the alignment of a principle axis in a crystal to an incident ion beam using Rutherford Back Scattering (RBS) is described in the above book by Chu et al, Chapter 8.2 Crystal Alignment Procedures, from page 225 to page 229. In this technique, the crystal is mounted on a goniometer when exposed to the ion beam. The goniometer allows the crystal to be rotated about an axis, which axis can itself be tilted, to a desired tilt angle relative to the incident beam direction, about a tilt axis which is made perpendicular to the beam direction. Back scattered ions are detected at a detector located in the plane common to the beam direction and the tilt axis.
The chapter describes how plots can be made on a polar diagram of minima in the back scattered intensity at a selected energy, as the crystal is rotated about the rotation axis when aligned at various tilt angles relative to the incident beam. Essentially, if a principle crystal axis of the crystal on the goniometer is accurately aligned to the axis of rotation of the goniometer, then minima in the back scattered intensity for a particular tilt angle, are symmetrically distributed about the polar plot at angles corresponding to the orientation in the crystal of crystal planes parallel to the principle crystal axis. Any deviation of the principle crystal axis from the axis of rotation of the goniometer, causes the minima to plot around the polar diagram in such a way that the offset in polar co-ordinates of the crystal axis relative to the axis of rotation can easily be measured.
In ion implanters it is known for the wafer holder to be mounted for rotation about an axis (twist axis) which is nominally perpendicular to the surface of a silicon wafer mounted on a holder. The wafer holder is also commonly adjustable in tilt, to change the angle that the wafer presents to the ion beam, about a tilt axis perpendicular to the nominal beam direction and to the twist axis. An example of a wafer holder of this kind is described in WO 99/13488 (Orion Equipment Inc.).
Examples of the invention provide a solution to the problem of accurately measuring, confirming or correcting the alignment of the process beam in the process chamber of an ion implantation system.
In its broadest aspect, the invention provides for the use of Rutherford Back Scattering (RBS) techniques for measuring, confirming or correcting the alignment of the process beam in the process chamber of an ion implantation system. The direction of an ion beam relative to a wafer holder in an implant chamber of an ion implanter can be calibrated by
a) mounting on the holder a crystalline material so that the crystal lattice of the crystalline material has a known nominal orientation relative to the holder,
b) directing the ion beam at the crystalline material on the holder,
c) effecting relative movement between the direction of the ion beam and the orientation of the holder,
d) monitoring the intensity of beam ions back scattered from the crystalline material during said relative movement to identify, from a corresponding minimum in said monitored intensity, at least one selected orientation of the crystal lattice of the crystalline material relative to the ion beam, and
e) using the relative orientation of the ion beam and the holder when said selected crystal lattice orientation is identified and the known nominal orientation of the crystal lattice to the holder to provide a calibration point for the direction of the ion beam relative to the wafer holder.
This technique uses the phenomenon mentioned in Chu et al whereby the intensity of back scattered beam ions falls away when the beam becomes aligned with a channelling plane or axis of the crystal. This is because beams tend to travel more deeply into the crystal and thus back scattered ions tend to be absorbed when passing through a greater thickness of crystal material in returning to the surface for detection.
Because the nominal orientation of the crystalline material relative to the holder is known the actual orientation between the ion beam and the holder at a minimum of the back scattered intensity corresponding to a selected channelling plane or axis of the crystal can be used as a calibration point to calibrate the direction of the ion beam relative to the wafer holder.
For a single measurement, the accuracy of the calibration point can correspond to the accuracy with which the orientation of the crystalline material relative to the holder is known. To compensate for uncertainty in the precise orientation of the crystalline material relative to the holder, the measurement can be repeated after reversing the orientation of the crystalline material on the holder by 180xc2x0 and then taking an average of the two measured calibration points.
Where the ion beam has a nominal direction and the wafer holder can tilt the wafer about a tilt axis perpendicular to the nominal beam direction and can rotate the wafer about a twist axis nominally perpendicular to said tilt axis, the component of the direction of the ion beam relative to the wafer holder in the plane perpendicular to the tilt axis can be calibrated by identifying a minimum in the monitored back scattered ion intensity when changing the tilt of the holder. In cases where the wafer holder cannot be tilted about an axis orthogonal to said tilt axis (and still perpendicular to the nominal beam direction), then it is necessary for the ion beam direction to be adjustable in a plane parallel to the tilt axis, so that the component of the direction of the ion beam relative to the wafer holder in said parallel plane can be calibrated by identifying a minimum in the monitored back scattered ion intensity when adjusting the beam direction in said plane.
In another arrangement, the alignment of the ion beam in the plane of the tilt axis can be checked using a crystalline material on the holder with a known crystal direction aligned with said twist axis, by tilting the holder to bring said crystal direction parallel to the plane of the beam parallel to the tilt axis and monitoring the back scattered ion intensity while rotating the holder about the twist axis. If the ion beam is misaligned in the plane of the tilt axis so as not to be parallel to the twist axis (and the known crystal direction) the back scattered ion intensity will be higher in the absence of perfect channelling of beam ions down the known crystal direction, and will vary as the holder is rotated as successive channelling planes become parallel to the beam alignment.
The invention also provides alignment measuring apparatus for fitting to an ion implanter of the kind having a process chamber having a chamber wall, an ion beam generator to produce the beam of ions for implantation, said beam having a nominal beam direction in said chamber, a wafer holder in said chamber to carry for ion implantation therein a planar crystalline wafer having a crystal axis nominally normal to the plane of the wafer, said wafer holder having a normal direction which is nominally normal to the plane of a wafer on said holder, a tilt mechanism for adjusting the tilt angle of said normal direction of the holder relative to said nominal beam direction about an axis perpendicular to said normal direction and to said nominal beam direction, and a twist mechanism for rotating the holder and any wafer thereon to selected rotational positions about an axis parallel to said normal direction, the alignment measuring apparatus comprising a scattered ion receiver adapted to be mounted on said chamber wall to receive beam ions back scattered from a wafer on the holder and a scattered ion current detector for determining the current of said back scattered beam ions received by said receiver.
The back scattered ion receiver and current detector can, when mounted on an ion implanter as described, be used for performing the calibration methods described above.
Preferably, the scattered ion receiver includes a filter to reject low energy secondary ions from the wafer and pass substantially only back scattered beam ions to said current detector. The scattered ion current detector may comprise an ion counter providing a value of said current corresponding to the count after a predetermined counting period.
The invention also contemplates more generally, other apparatus for performing the described methods.